Long-term trends in prairie vegetation at three national parks: 1998–2022

Leis, Sherry; Morrison, Lloyd

doi:10.36967/2302359

The Heartland Inventory and Monitoring Network has monitored plant communities in National Parks since 1998. Three of those parks in the northern tier of the network—Herbert Hoover National Historic Site (NHS), Homestead National Historical Park (NHP), and Pipestone National Monument (NM)—have reconstructed tallgrass prairie communities and were sampled concurrently in 2022. In this combined report, we describe similarities and differences among the three parks related to current vegetation and trends. Climatically, Herbert Hoover NHS and Homestead NHP have similar temperature profiles, but Homestead NHP is drier. Pipestone NM differs from the other two parks in temperature and precipitation. Long-term climatic signals for major drought events varied by park, and moisture at Herbert Hoover NHS is likely to increase with climate change. Precipitation shifts could influence vegetation trends in the future requiring action such as flood mitigations, wildfire protections during prolonged drought, and consideration for species additions to adapt to new climate profiles. Plant composition was similar for Herbert Hoover NHS and Homestead NHP while the combination of plant species and abundances at Pipestone NM was different from the other two parks, especially within the Sioux Quartzite community type. There appeared to be some compositional shifts over time for Homestead NHP. That trend was supported by greater species turnover at Homestead NHP than the other two parks. The reconstruction at Homestead NHP is older than that of Herbert Hoover NHS and possibly Pipestone NM, but we are unable to determine the underlying causes of species changes. Quality assessment procedures provided a moderate level of confidence in our data with respect to botanist agreement on identifications. We met our goal of about 80% agreement in species composition. Cover class agreement was greater than 70%, with less than 4% of observations off by more than one class. Native species richness trends varied by park. Herbert Hoover NHS continues to gain native species while Pipestone NM is losing species. Species richness at Homestead NHP did not exhibit a directional trend. Pipestone NM tended to have less cover of forbs and grasses than the other two parks, reflecting the more complex geological landscape with surface rock. Grass abundance appears to be declining from baseline years for all three parks. Grass-like and fern guilds are much less abundant in all the parks than other plant guilds. Woody plants in 2022 were similar at Herbert Hoover NHS and Homestead NHP in terms of mean cover and heterogeneity across the prairies, but tree encroachment into the grasslands is a potential concern. Pipestone NM generally had fewer woody plants (including tree seedlings) in 2022, but the amount varied through time. Canopy closure, measured for the first time in 2022, was present in all three parks, but was greatest at Homestead NHP. Nonnative plant cover was the greatest at Pipestone NM, but current abundance at Herbert Hoover NHS was greater in 2022 than baseline years. Homestead NHP consistently had little nonnative plant cover since 2005. Abundance (% cover) of two nonnative grasses of concern—Kentucky bluegrass (Poa pratensis) and smooth brome (Bromus inermis)—differed by park and year. Recent increases in Kentucky bluegrass abundance at Herbert Hoover NHS might reflect changes in management. Although these parks have individual differences in climate as well as in timing and use of management actions, they share similar histories of cultivation and reconstruction. Efforts to restore or reconstruct portions of the prairie at Pipestone NM were unique among the three parks. Future analyses might continue to combine Herbert Hoover NHS and Homestead NHP but examine Pipestone NM separately; the unique geologic history and differences in vegetation communities at Pipestone NM may make individual assessment a better option. However, comparing trends in guilds of concern, such as woody and nonnative plants, across all three parks can be helpful for gauging success with management tools in light of regional changes in climate.

Long-term trends in prairie vegetation at three national parks: 1998–2022

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